This invention relates to wave reflectors and, in certain exemplary embodiments, Bragg mirrors used in conjunction with piezoelectric resonators.
Piezoelectric resonators are used for a number of purposes but are primarily used in the electronics industry for signal filtering and reference oscillators. These resonators are commonly referred to as FBAR (film bulk acoustic resonators) or BAW (bulk acoustic wave resonators). The resonator is preferably acoustically isolated from the mechanical substrate, e.g. a silicon wafer. This can be accomplished by an air gap for a FBAR or a Bragg mirror for a Solidly Mounted Resonator (“SMR”). Bragg mirrors have been developed in both microwave and optical applications to reflect energy waves.
As is well known to those skilled in the art, a Bragg mirror typically comprises alternating high and low acoustic impedance materials that are each ideally approximately one-quarter the wavelength (λ/4) thick at the operating frequency of the resonator. Adjacent high and low acoustic impedance layers are generally referred to as “bi-layers”, and are approximately one-half the wavelength (λ/2) at the operating frequency of the resonators. These devices are well documented in the literature. See, for example, W. E. Newell, “Face-mounted piezoelectric resonators,” in proc. IEEE vol. 53, June 1965, pp. 575-581; L. N. Dworsky and L. C. B. Mang, “Thin Film Resonator Having Stacked Acoustic Reflecting Impedance Matching Layers and Method,” U.S. Pat. No. 5,373,268, Dec. 13, 1994; K. M. Lakin, G. R. Kline, R. S. Ketcham, and J. T. Martin, “Stacked Crystal Filters Implemented with Thin Films,” in 43rd Ann. Freq. Contr. Symp., May 1989, pp. 536-543; and R. Aigner, J. Ella, H.-J. Timme, L. Elbrecht, W. Nessler, S. Marksteiner, “Advancement of MEMS into RF-Filter Applications,” Proc. of IEDM 2002, San Francisco, Dec. 8-11, 2002, pp 897-900, all incorporated herein by reference.
Briefly, a Bragg mirror, also referred to as a “Bragg reflector” or a “quarter-wave mirror” is a structure which consists of an alternating sequence of layers of two different transmissive materials, with each transmissive layer thickness corresponding to one quarter of the wavelength for which the mirror is designed. Since the wavelength through a material is inversely proportional to the impedance of the material, a quarter-wavelength thickness of a low impedance material will be thicker than a quarter-wavelength thickness of a high impedance material. The latter condition holds for normal incidence.
The operation principle can be understood as follows. Each interface between the two materials contributes a Fresnel-type reflection. For the design wavelength, the optical path length difference between reflections from subsequent interfaces is one half the wavelength; in addition, the reflection coefficients for the interfaces have alternating signs. Therefore, all reflected components from the interfaces interfere constructively, which results in a strong reflection. The achieved reflectivity is determined by the number of bi-layers and by the refractive index contrast between the layer materials. The reflection bandwidth is determined mainly by the index contrast.
FIG. 1 illustrates a resonator assembly 10 including a substrate 12, a Bragg mirror 14 and a standard piezoelectric resonator 16. The substrate 12 can be, for example, a silicon wafer 18 covered with a low impedance (“Z”) layer 20, such as an oxide layer. It is to be understood with respect to the example of an acoustical Bragg mirror that the impedance of a layer is its acoustical impedance, and not another type of impedance such as electrical impedance.
The Bragg mirror 14 in this example has two bi-layers 22 and 24. Bi-layer 22 includes a low impedance (“LO-Z”) layer 22a and a high impedance (“HI-Z”) layer 22b. Similarly, bi-layer 24 includes a LO-Z layer 24a and a HI-Z layer 24b. The HI-Z layers have a relatively high impedance to compression (“acoustical”) waves, and usually include a metal such as aluminum or tungsten. The LO-Z layers have relatively low impedance to acoustical waves, and can be, for example, silicon oxide or polymer layers.
The thickness of each layer of the bi-layers 22 and 24 is conventionally set equal to a quarter-wavelength (λL/4) of the incident wave that is to be reflected. That is, each of layers 22a, 22b, 24a and 24b are a quarter-wavelength thick. Since wavelengths are dependent upon the impedance of the material through which they are traveling, the LO-Z layers are thicker than the HI-Z layers. The alternate layers have the characteristics that each layer shows a high contrast in impedance with the adjacent layers. For microwave applications this is electrical impedance, and for BAW/SMR applications it is the acoustic impedance Z, which is a function of the material density and mechanical characteristics E (Young Modulus) and ν (Poisson Ratio).
The high-contrast bi-layers, each having two layers set to a quarter-wavelength of the incident wave, create a highly efficient mirror for reflecting the incident wave. In applications with piezoelectric resonators, such as resonator 16, this means that the acoustical energy created by the resonator is reflected back to the resonator and is not absorbed into the substrate 12. This allows the resonator to operate more efficiently than if it were supported directly on the substrate.
It should be noted that FIG. 1 illustrates a Bragg reflector having two bi-layers, each tuned for the same wavelength. This is because each bi-layer will reflect only a certain percentage of the acoustical energy. For example, if a bi-layer reflects 90% of the incident wavelength, then two bi-layers will theoretically reflect 99% of the incident wavelength. Depending upon the application, prior art Bragg reflectors can have a single bi-layer or two or more bi-layers, all of the same thickness; where the bi-layer is composed of two layers, each ¼ of the wavelength of the acoustic wave to be reflected.
A problem not addressed by the majority of the prior art is that of reflecting multiple frequencies of acoustic waves. That is, the Bragg mirrors of the majority of the prior art efficiently reflect only a specific wavelength which is twice times the thickness of the Bragg mirror bi-layer (i.e. four times the thickness of each layer of the bi-layer). Of course, by “specific wavelength” it is meant the principal wavelength and wavelengths within a relatively narrow range, e.g. ±20%, of the principal wavelength.
Some devices, such as piezoelectric resonators, develop acoustic waves at other than a primary wavelength. For example, piezoelectric resonators develop both a principal acoustic wave and a “shear” wave which is approximately ½ the wavelength of the principal wave. Bragg mirrors of the majority of the prior art do not effectively reflect other wavelengths other than those to which they are tuned, such as the wavelength of harmonics.
United States Patent Application 20050200433, published Sep. 15, 2005, discloses a bulk acoustic wave filter and method for eliminating unwanted side passbands, wherein a bulk acoustic wave (BAW) filter is fabricated from thin film bulk acoustic wave resonators and a method eliminates unwanted side passbands. This BAW filter comprises a substrate a resonator section and an acoustic mirror section. Further it comprises a detuning component positioned in the resonator section to provide precise passband characteristics and an additional detuning component in the acoustic mirror section to suppress unwanted side-passband characteristics.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.